232 posts

### amount of material present in kg at any instant in a close ckt ball mill

I WANT TO CALCULATE THE MATERIAL PRESENT IN KG IN A BALL MILL , IF THE FRESH FEED IS 100 TPH & 300 TPH IS THE MILL OUTLET PRODUCT.

IT IS A CEMENT MILL

BY THIS I SHALL BE ABLE TO CALCULATE THE THICKNESS OF MATERIAL LAYER INSIDE THE MILL OR IN OTHER WORDS THE FILLING % OF MATERIAL ALONE IN A BALL MILL EXCLUDING GRINDING MEDIA.

IN MY OPENION I REQUIRE THE RETENTION TIME OF MATERIAL INSIDE TRHE MILL TO GET THIS VALUE.

PLEASE DO COMMENT

SIDDU

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79 posts

### re amount of material present in kg at any instant in a close ckt ball mill

Dear

The material retaintion time in a ball mill depends on the tons of ball and total material fed to the mill (fresh feed +separator’s rejects). It can be approximated using the following expression:

**T(minutes) = 6.7 x (F/P)**

Where:

F = total tons of grinding bodies in the mill

P = Mill’s production (total dry feed material) (ton/hr)

It is also possible to calculate the tons of resident material inside the mill. This means, the tons of material that would remain if the mill if mill is suddenly shut down.

**M(ton) = P x (T(minutes)/60)**

For example, a mill that produces 100 ton/hr with a circulation factor of 2 and 250 tons of ball, retains material for an average period of:

6.7 x (250/100) = 16.75 minutes

The material inside the mill will be

100 x 16.75/60 = 27.91 mt

Practically checking of retaintion time in a mill :

**The residence time of material is checked practically by using some **
**fluorescent ****material . It is added to the feed material at mill inlet. The mill outlet sample is collected in every 2 to 3 minutes. The time when the concentration of the fluorescent is high, the time is noted.
**

**IT IS DISCUSSED EARLIER IN THE FORUM.
**

The retention time is calculated from time inlet to time outlet.

232 posts

### re amount of material present in kg at any instant in a close ckt ball mill

Dear Chari

In my openion if the circulation factor is 2 with 100 tph product than retention time should be

6.7*250/300 = 5.58 minutes

Corresponding material inside the mill

5.58*300/60 = 27.9 t

It is ok ,matching with the end result of 27.9 t.

Now how much power the mill takes to rotate this 27.9 t?

How can I calculate the material layer thickness inside mill if the density of the material is 1.2 t/m3 & mill dimentions are known with known media tons?

79 posts

### re amount of material present in kg at any instant in a close ckt ball mill

Sorry for the delay in reply, You are right as I have not considered the return load by mistake. General retaintion time in a mill with 70% rpm of critical speed is taken as 5 to 7 minutes for Low L/D mills.

If the total media is 250 mt in the mill and BD of the media taken as 4.55t/m3 , the specific gravity of steel balls is taken as 7.8t/m3 so the volume of voids in the media is

250 x ( 1/4.55 - 1/7.8) = 22.9 m3

if BD of mix is 1.2 than material in the mill without increase in the height of the media level is 22.9x1.2 = 27.5 mt.

if the retaintion material is high then there will be an increase in volume of material inside the mill.

chari